The present invention relates to a variable displacement pump which serves as a source for supplying the hydraulic pressure to a hydraulic device such as an automotive power steering apparatus, and more particularly, to the variable displacement pump having discharge controlled by changing the volume of a pump main part.
Typically, the variable displacement pump comprises a rotor rotated by a driving shaft, vanes mounted to the outer periphery of the rotor to be movable radially, and a cam ring eccentrically arranged on the outer periphery of the rotor and having a roughly circular inner-peripheral surface. Due to the rotor and cam ring being offset to each other, when the rotor rotates, the vanes move radially in accordance therewith with the front ends making slide contact with the inner-peripheral surface of the cam ring. Thus, the volume of pump chambers each formed between circumferentially adjacent vanes is increased or decreased continuously.
Some variable displacement pumps further comprise a mechanism for variably controlling the volume of the pump chambers. With such variable displacement pump, the cam ring is swingably arranged on the outer periphery of the rotor, and has both sides slidably closed by closing members. The volume of the pump chambers can arbitrarily be adjusted by changing the amount of eccentricity between the rotor and the cam ring through adjustment of oscillation of the cam ring. The cam ring is swingably arranged inside a roughly elliptic adaptor ring. The inside of the adaptor ring has first and second working chambers defined in first and second swing directions of the cam ring.
Suction and discharge passages are connected to the suction and discharge areas within the cam ring, respectively. An orifice is provided to the discharge passage. The first working chamber is constructed to introduce therein working fluid having pressure controlled by a control valve. The second working chamber is constructed to accommodate a spring for biasing the cam ring to the first working chamber, and always introduce therein low-pressure working fluid from the suction passage. The control valve is operated in response to a pressure difference between the upstream and downstream sides of the orifice to control working fluid introduced into the first working chamber in accordance with the pressure difference.
With the above variable displacement pump, the second working chamber has suction-side low pressure maintained at all times, whereas the first working chamber has pressure controlled in accordance with the pressure difference between the upstream and downstream sides of the orifice. Thus, an inconvenience can be eliminated that, under the conditions of low pump rotational speed where an increase in the flow rate of working fluid is desired (wherein the cam ring is maximally displaced to the first working chamber to maximize the amount of eccentricity), high-pressure working fluid leaks to the low-pressure side from the first working chamber through clearances there around.
The closing member disposed at the side of the cam ring is formed with a suction port which opens to the suction area of the cam ring and a discharge port which opens to the discharge area of the cam ring. The suction and discharge ports are connected to the suction and discharge passages, respectively. This closing member is also formed with a low-pressure introduction hole extending axially to connect the second working chamber and the suction passage, through which low-pressure working fluid of the suction passage is always introduced into the second working chamber.